A magnetoresistive element having a ferromagnetic tunnel junction is also called a magnetic tunnel junction (MTJ) element, and as its write system, a write (spin transfer torque write) system using spin momentum transfer (SMT) is proposed.
There has been assumed that a so-called perpendicular magnetization film having a magnetization easy axis in the film plane perpendicular direction is used as a ferromagnetic material making the magnetoresistive element. Since when magnetocrystalline anisotropy is used in a perpendicular magnetization structure, shape magnetic anisotropy is not used, an element size can be further reduced than in the in-plane magnetization structure. Further, since dispersion in the magnetization easy direction can be also reduced, a material having large perpendicular magnetocrystalline anisotropy is employed in order to realize both smaller element size and lower switching current while a thermal stability is maintained.
Material systems used for the perpendicular magnetization film include L10 ordered alloy system (such as FePt and CoPt), multilayer film (Co/Pt, Pd) system, hcp system (such as CoCrPt) and RE-TM system (such as Tb—CoFe).
Typically, a switching current for magnetization reversal by the spin transfer torque write system depends on saturation magnetization Ms and a magnetic relaxation constant α of a storage layer. Thus, it is important to reduce the saturation magnetization Ms and the magnetic relaxation constant α in order to switch the magnetization of the storage layer by low-current spin transfer torque.
Herein, the saturation magnetization. Ms can be reduced by adjustment of magnetic material's composition, addition of nonmagnetic element or the like. However, the reduction in the saturation magnetization Ms should not cause adverse effects on other properties.